1. Field of the Invention
The present invention relates to an Si product, typically an Si substrate, used in the production of various devices such as a semiconductor device, an X-ray mask and a micro-mechanism, as well as for use as a light-transmitting substrate, and also to a method of processing such an Si product. More particularly, the present invention is concerned with an Si product or substrate which is obtained by forming at least one porous Si region in an Si body and then removing such porous Si region, as well as to a processing method for processing such an Si product.
2. Related Background Art
Studies have been made in recent years as to, for example, micro-mechanisms which are produced by processing a bulk Si by various etching methods such as chemical etching, RIE (reactive ion etching) and electrolytic grinding.
Chemical etching is a method in which an Si substrate is partly covered by a mask of a resist, Si.sub.3 N.sub.4 or SiO.sub.2 and is immersed in an etchant so that the Si substrate is selectively etched at portions which are lot covered by the mask.
RIE (reactive ion etching) is a method in which an Si substrate is partly covered by a mask of a resist, Si.sub.3 N.sub.4 or SiO.sub.2 and is subjected to a reactive ion atmosphere so that the Si substrate is selectively etched at portions which are not covered by the mask.
Electrolytic grinding is a method in which an electrolysis is conducted in an HF solution or a KOH solution by employing Si as an electrode, so as to grind the Si electrode. In a method known as anodic etching which is a kind of the electrolytic polishing, an electrolysis is conducted in a thin HF solution using Si and platinum or gold as an anode and a cathode, respectively, so that the Si is etched as a result of the electrolytic reaction taking place on the above.
Porous Si was discovered by Uhlir et al. in 1956 in the course of study of electrolytic grinding of semiconductors (A. Uhlir, Bell Sys. Tech. J. vol. 35,333 (1956)).
Unagami et al. have studied Si dissolving reaction of Si in anodization and found that holes are essentially required in anodic reaction of Si in HF solution. Unagami et al. also report that the dissolving reaction is as follows (T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)): EQU Si+2HF+(2-n)e+.fwdarw.SiF.sub.2 +2H.sup.+ +ne.sup.- ( 1) EQU SiF.sub.2 +2HF.fwdarw.SiF.sub.4 +H.sub.2 ( 2) EQU SiF.sub.4 +2HF.fwdarw.H.sub.2 SiF.sub.6 ( 3)
or, alternatively, EQU Si+4HF+(4-.lambda.)e.sup.+ .fwdarw.SiF.sub.4 +4H.sup.+ +.lambda.e.sup.-( 4) EQU SiF.sub.4 +2HF.fwdarw.H.sub.2 SiF.sub.6 ( 5)
wherein e.sup.+ and e.sup.- respectively represent a hole and an electron, n and .lambda. respectively represent the number of holes necessary for dissolving one Si atom. It is reported that a porous Si is formed on condition of n&gt;2 or .lambda.&gt;4.
Thus, production of porous Si essentially requires holes. This means that N-type Si is easier to change into porous Si than P-type Si is. It is known, however, N-type Si can also be changed into porous Si by injection of holes (R. P. Holmstrom and J. Y. Chi, Appln. Phys. Lett., vol. 42,386 (1983)).
While single crystal Si generally has a density of 2.33 g/cm.sup.3, the density of porous Si can be varied within the range of 1.1 to 0.6 g/cm.sup.3 when the density of HF solution is varied between 50 and 20%.
A transmission-electroscopic observation shows that a porous Si layer has micro-pores of about 600 .ANG. in mean diameter, and that single crystallinity is still maintained despite that the density is reduced to less than half than that of the single crystal Si. It is therefore possible to form a single crystal Si layer on the porous layer by epitaxial growth.
In general, oxidation of a single crystal Si causes an expansion of the Si to a size which is about 2.2 times as large than that of the original single crystal Si. It is possible to restrain the expansion by controlling the density of the porous Si, which makes it possible to prevent warping of an Si substrate, as well as cracking which may be introduced into the remaining single crystal surface layer.
Representing the density of the porous Si by A, the volumetric ratio R between the volume of the single crystal Si after oxidation to the volume of the porous Si is represented by the following formula: EQU R=2.2.times.(A/2.33) (6)
When no expansion is caused by oxidation, i.e., when R=1 is met, the density A of the porous Si is 1.06 (g/cm.sup.3). This means that expansion of the single crystal Si is suppressed by selecting the density of the porous Si to be 1.06.
In general, a porous layer has a large void so that its density is as small as half or below that of the non-porous structure. The porous structure, therefore, has a surface area which is much greater than that of the non-porous structure. For this reason, a porous layer exhibits a much greater etching rate than an ordinary single crystal layer.
A description will now be given of an operation for etching a porous Si.
In the current technique, in almost all cases, porous Si products as produced are directly subjected to a subsequent step, e.g., an epitaxial growth process. In other words, no processing is effected on the porous Si, due to difficulty encountered in processing or removing porous Si with high degrees of controllability. As a matter of fact, no report has been made which would show that etching of porous Si can be done with good controllability.
In general, it is possible to obtain an oxidized porous Si equivalent in quality to single crystal Si oxide film by adjusting condition of anodization such that the porosity P, which is expressed by the following formula (7), falls within the range of between 30 and 55%: EQU P=(2.33-A)/2.33 (7)
The porosity P also can be expressed as follows: EQU P=(m1-m2)/(m1-m3) (8)
or EQU P=(m1-m2)/.rho.At (9)
m1: overall weight before anodization PA1 m2: overall weight after anodization PA1 m3: overall weight after removal of porous Si PA1 .rho.: density of single crystal Si PA1 A: area changed into porous state PA1 t: thickness of porous Si
In some cases, it is not possible to exactly calculate the area changed into porous Si. In such cases, the formula (8) is advantageously employed but the formula (8) essentially requires etching of the porous Si for the purpose of measurement of the value of m3.
When an epitaxial growth is effected on a porous Si, any stress which is generated during hetero-epitaxial growth can conveniently be absorbed by the specific structure of porous Si, thus suppressing generation of defects. Obviously, the porosity of the porous Si is an important factor for achieving such a stress relieving effect. Measurement of porosity is therefore essential in order to successfully carry out epitaxial growth on porous Si.
Two types of etching methods are known as methods of etching porous Si.
The first method employs an aqueous solution of NaOH as the etchant (G. Bonchil, R. Herino, K. Barla, and J. C. Pfister, J. Electrochem. Soc., vol. 130, no. 71611 (1983)) while the second method employs, a solution which is capable of etching non-porous Si as the etchant.
The etchant used in the second method is typically a fluoro-nitric acid type etchant. When such an etchant is used, the etching process can be represented as follows: EQU Si+2O.fwdarw.SiO.sub.2 ( 10) EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +H.sub.2 O (11)
Thus, Si is oxidized by nitric acid into SiO.sub.2 which is then etched by hydrofluoric acid, whereby Si is etched.
Various other etchants can be used for etching non-porous Si, such as etchants of ethylene diamine type, KOH type and hydrazine type, besides fluoro-nitric acid type etchant mentioned before.
Thus, selective etching for removing porous Si essentially requires that the etchant employed can etch porous Si but not non-porous Si.
These known methods, however, involve the following problems to be solved.
For instance, the bulk etching process essentially requires a mask because there is no difference in the material between the area to be etched and the area which is not to be etched.
Chemical etching often allows a lateral over-etching. In addition, a surface of low-etching rate appears in an anisotropic etching. It is therefore impossible to form an etched region, e.g., a hole, purely by walls perpendicular to the material plane. Furthermore, the form of the etched region varies according to the plane azimuth of the Si substrate.
The RIE (reactive ion etching) can effect etching in the direction perpendicular to the material surface but is almost unable to perforate an Si layer which is as thick as several hundreds of microns to several millimeters.
In electrolytic grinding method which employs an electrical current, it is not allowed to provide the surfaces of the Si substrate with an insulation layer or a semiconductor layer other than the mask.
The etching method which uses a fluoro-nitric acid etchant undesirably allows non-porous Si to be etched, as well as porous Si which is intended to be etched.
Finally, the known method for selectively etching a porous Si by means of aqueous solution of NaOH inevitably suffers from adsorption of Na ions on the etched surface. Deposition of Na ions is a major cause of contamination with impurities and produces undesirable effects such as formation of an interface level and, therefore, must strictly be avoided.